Methods of electroplating zinc and cyanide free or low cyanide zinc plating baths therefor

ABSTRACT

Cyanide-free or low cyanide alkaline zinc plating solutions containing a soluble silicate and a process of plating bright zinc from cyanide-free or low cyanide containing alkaline zinc plating solutions containing a soluble silicate.

United States Patent Kessler et al.

[ Dec. 24, 1974 METHODS OF ELECTROPLATING ZINC AND CYANIDE FREE OR LOW CYANIDE ZINC PLATING BATHS THEREFOR Inventors: Richard B. Kessler, Jericho; Fred 1.

Nobel, Roslyn, both of NY.

Assignee: Lea-Ronaf, Inc., Freeport, N.Y.

Filed: July 9, 1973 Appl. No.: 377,817

US. Cl. 204/29, 204/55 R, 204/55 Y Int. Cl. C23b 5/10, C23b 5/46, C23b 5/62 Field of Search 204/55 R, 55 Y, 43 Z, 44,

References Cited UNITED STATES PATENTS 8/1909 Van Winkle 204/55 R Primary Examiner-G. L. Kaplan Attorney, Agent, or FirmPennie & Edmonds [57] ABSTRACT Cyanide-free or low cyanide alkaline zinc plating solutions containing a soluble silicate and a process of plating bright zinc from cyanide-free or low cyanide containing alkaline zinc plating solutions containing a soluble silicate.

5 Claims, N0 Drawings METHODS OF ELECTROPLATING ZINC AND CYANIDE FREE OR LOW CYANIDE ZINC PLATING BATI-IS THEREFOR BACKGROUND OF THE INVENTION Basically there are three different types of alkaline zinc plating solutions in commercial use today. These are generally referred to as high cyanide containing alkaline zinc solutions, low cyanide containing alkaline in solutions and cyanide-free alkaline Zinc solutions.

Conventional high cyanide containing alkaline zinc solutions are based upon zinc cyanide, a large amount of free sodium cyanide, sodium hydroxide and certain brightening agents and have been in use for many years.

These high cyanide containing alkaline zinc plating solutions can successfully be used in substantially all areas throughout the United States without modification to account for the quality of the water available in any given area. Water hardness and the presence of other impurities in the water used to prepare the plating solutions, and also in the rinse waters used to prepare the work for plating, do not interfere with the ability of the solutions to plate bright zinc deposits.

Cyanide-free alkaline zinc plating solutions in commercial use today are based upon sodium zincate, sodium hydroxide and certain brightening agents. Low cyanide containing alkaline zinc plating solutions in commercial use today are similar to or the same as the cyanide-free alkaline zinc solutions but also include a minor amount of free sodium cyanide.

When cyanide free or low cyanide containing alkaline zinc plating solutions are used to plate bright zinc, impurities in the water used for rinsing the work in preparation for plating and in the water used in the plating process itself has a profound effect. When the water hardness and/or other impurities are high it is often difficult, if not impossible, to obtain bright deposits. The degree of difficulty in obtaining bright zinc deposits has been found to depend upon the degree of water hardness and/or the concentration of other impurities in the water. The brightness obtainable will also naturally be dependent on the relative ability of the particular brightener used in a bath for giving bright zinc deposits.

Cyanide-free or low cyanide containing alkaline zinc plating solutions are obviously preferred for commercial use due to the difficulty of disposing of the high cyanide containing alkaline zinc solutions in compliance with pollution laws and regulations.

In preparing metal for zinc plating by conventional methods a cleaning and rinsing operation of the metal to be plated is employed prior to the actual plating operation. Experiments have shown that if the rinse water is hard or contains a high amount of calcium and magnesium contaminants that a film of water is formed on the metal object to be plated which will contain these contaminants as well as any others that may be present in the rinse water. When these wet parts are then immersed into a cyanide-free or low cyanide containing alkaline zinc plating solution, the calcium, magnesium or other insoluble impurities appear to precipitate on the metal parts to be plated as soon as they are wet by the zinc plating solutions. The presence of this precipitate on the metal parts to be plated causes dull and smeared deposits. Although the applicants have no proof that the precipitation of these contaminants on the metal part to be plated is the cause of the dull and smeared deposits, the fact is that the dull and smeared deposits result when metal objects to be plated are rinsed in hard water or water containing impurities seems to be fairly conclusive, since, when the same metal parts to be plated are rinsed in distilled water and zinc plated thereon by the very same zinc plating solution, much brighter deposits are obtained than when the same metal objects are rinsed in hard water or water containing contaminants.

In high cyanide zinc plating solutions this problem is not apparent probably because the high cyanide content of these solutions is able to solubilize or peptize the calcium, magnesium or other impurities present therein so that no precipitate is formed and bright deposits are obtained when the metal objects to be plated are rinsed in both distilled and hard water.

In order to overcome these problems when using hard water or water containing other impurities in the preparation of cyanide free or low cyanide containing alkaline zinc plating solutions, it has been proposed to add chelating agents or complexing agents to the zinc plating solutions and/or to the rinse water just prior to the zinc plating operation to solubilize these impurities. Examples of such chelating or complexing agents that have been proposed include ethylenediaminetetraacetic acid salts, sodium gluconate, sodium potassium tartrate and so forth. These materials appear to work fairly satisfactorily in producing bright zinc deposits but they introduce another problem into the zinc plating process because they also chelate or complex metal impurities contained therein and cause them or permit them to be built up in the bath. These metallic impurities such as iron, copper and so forth thus build up to a point where they cause the zinc deposit to become dull or cause problems in subsequent zinc brightdipping and chromating operations that normally follow bright zinc plating. In low cyanide containing zinc plating solutions the metallic build-up is less of a problem because of the presence of a small amount of cyanide, but, nevertheless, the problem still exists.

SUMMARY OF THE INVENTION This invention relates to the incorporation of a small amount of a soluble silicate compound in cyanide-free or low cyanide containing alkaline zinc plating solutions which obviates the above discussed disadvantages in plating bright zinc from cyanide-free or low cyanide containing alkaline zinc plating solutions regardless of the hardness of the water and/or the impurities contained in the water used to formulate the plating solutions.

The invention also relates to the use of a soluble silicate compound in the rinse water used to rinse the object to be plated in the cyanide-free or low cyanide plating solution to obtain bright or semi-bright zinc deposits.

DETAILED DESCRIPTION OF THE INVENTION Various soluble silicate compounds can be used to accomplish the objects of this invention. Some examples include water glass Na O'XSiO (X=3-5); sodium disilicate Na Si O sodium metasilicate Na SiO and sodium orthosilicate Na SiO,; as well as other metal salts thereof. Other soluble silicate compounds can of course be used in accordance with this invention as will be apparent to those skilled in the art. The term soluble silicate compound is used herein to denote a silicate compound that is soluble in water or soluble in the alkaline environment of the particular zinc plating bath in which it is being used.

The soluble silicate compounds can be used singly or in combination in the cyanide-free or low cyanide containing zinc plating solutions and/or to the rinse water in which the parts to be plated are rinsed just prior to the zinc plating operation. Thus the soluble silicate compounds can be added to the water used to rinse the article just prior to the zinc plating operation and eliminating the necessity of adding the silicate to the plating solution during the plating operation. The water used in the plating solution can thus be hard or contain other impurities. Similarly, the soluble silicate compound can be added to the plating solution and not to the rinse water. The soluble silicate compound can be added to both the rinse water and the plating solutions if desired.

The soluble silicates can be added to any alkaline cyanide-free or low cyanide containing zinc plating solutions together with any other additives that might be employed including any brightener compounds capable of producing bright zinc deposits.

The amount of the soluble silicate compounds that can be added to the rinse waters or plating solutions will of course vary depending upon the particular soluble silicate compound being employed, but more particularly, depending upon the hardness of the water and the content of other impurities being used as the rinse water or in the zinc plating solution. The amount of soluble silicate is not at all critical and an excess is in no or alkaline amine-epichlorohydrin reaction products containing recurring tertiary or quaternary amine groups having a molecular weight above about 250 and a wide variety of other brightening compounds such as aldehydes, ketones, thiourea, organic acid salts and alkaline amines such as ethylenediamine, triethylenetetramine as well as those disclosed in Winters US. Pat. No. 2,791,554 and the Kampi US. Pat. No. 3,655,534

or mixtures thereof.

As previously discussed the applicants have no proof of the particular impurity or impurities in the water which causes deposition of dull zinc deposits even in the presence of conventional brightening agents which would normally produce a bright zinc deposit, nor are the applicants aware of the exact mechanism which takes place in the rinse water or plating solution with the soluble silicates which overcomes this problem and produces bright zinc deposits. Some examples of waters that would normally result in the production of dull or smeared zinc deposits despite the presence of a zinc brightener in the zinc plating baths, but which plate bright zinc in the presence of a soluble silicate compound are those of Los Angeles. This same problem also exists in other parts of the country, such as in the Middle West, including the states Michigan, Indiana and Illinois. Bright zinc plates are obtained using the process such as set forth in Example I without the addition of soluble silicate in other areas, such as New York City. An average chemical analysis of Los Angeles water sources for the fiscal year l97l-l 972 is given below. The analysis of these waters have also shown a variation of from 25 to 50% from its average.

CHEMICAL ANALYSES FOR MAJOR LOS ANGELES WATER SOURCES AVERAGE FOR FISCAL YEAR 1971-1972 LOS ANGELES LOS ANGELES METROPOLITAN WATER DISTRICT OWENS RIVER RIVER (Palos Vcrdcs SUPPLY AQUEDUCT CONDUIT (Upper Feeder) Reservoir) Specific Conductance (KXIO pH 320 621 1277 I243 Dissolved Residue (Calculated) 199 384 802 790 Total Hardness (CaCO 82 200 151 230 Calcium (Ca) 24 57 36 56 Magnesium (Mg) 5.1 14 15 22 Sodium (Na) 33 49 2l6 177 Potassium (K) 3.6 3.8 4.9 4.8 Alkalinity (CaCO 113 141 I26 I23 Sulfates (S0 28 110 338 341 Chlorides (Cl) 14 37 105 I05 Nitrates (NO;) 0.8 6.1 2.1 1.6 Silica (SiO 22 21 8 8 Iron (Fe) 0.05 0.02 0.01 0.()1 Boron (B) 0.41 0.37 0.18 0.23 Fluoride (F) 0.56 0.48 0.36 0.36

All chemical results in parts per million.

way harmful to the plating of bright zinc deposits. Generally in practice the amount added is not even measured and if the plating isnt sufficiently bright a little more of the soluble silicate compound is added. Generally, an amount of from about 0.001 to 2 ounces per gallon are sufficient to produce bright zinc deposits with most of the hard water available in the United States today.

As previously discussed, any brightening agent or compound capable of producing bright zinc deposits can be used in accordance with this invention. Some examples of brightening agents include alkaline amine One reason that the applicants cannot explain the mechanism taking place which results in the production of bright zinc deposits from such waters or pin point any particular impurity or contaminant in the water which causes the dull and smeared deposits in the absence of the presence of the soluble silicate is that conventional water softeners have no or little effect on the quality of the zinc deposits. For example, carbonates, phosphates, such as sodium phosphate and pyrophosphates and borates when added to the bath have little or no effect in overcoming the dull or smeared zinc deposits.

It is noted that the above analysis of Los Angeles water sources shows the presence of silica (SiO ranging from 22 to 8 parts per million. Silica can be present in water in various forms including dissolved silicates or silicic acid or colloidal silica. It is not known in what form the silica is present in these Los Angeles waters. An average analysis of New York City water for 1972 in parts per million as supplied by New York City is as follows:

Catskill-Delaware Hardness as CaCO Calcium (Ca) Magnesium (Mg) Sodium (Na) Potassium (K) Alkalinity (CaCO Sulfates (S Chlorides (Cl) Nitrates (N0 Silica (SiO Iron (Fe) Boron (B) Fluoride (F) All of the reported impurities in New York City waters are substantially less (including SiO than in the Los Angeles waters except iron and fluoride so it is difficult to determine exactly what impurities cause the dull plating problem when using Los Angeles or Mid- West waters. It is known, however, that the adidition of a small amount of soluble silicate to these Los Angeles of Mid-West waters overcomes the problem of streaky and smeared zinc depositions.

The analysis of the above waters is of course not complete since they contain many other impurities of a more minor nature including many other metal salts as well as organic matter which also may be the cause of the dull plating problems with certain waters but the applicant believes that the problem is caused by one or more of the impurities reported above in the Los Angeles water analysis.

EXAMPLE I A zinc plating bath was prepared by mixing the following ingredients in the conventional manner:

Zinc metal 1.6 oz./gal. Sodium hydroxide l6 oz./gal. A condensation reaction product of epichlorohydrin and dimethylaminopropylamine, having a molecular weight above 250 6 ccJgal. Anisic aldehyde bisulfite 0.04 gm./gal.

The water used to formulate the above zinc plating solution was Los Angeles water as described above. A steel panel was cleaned in the conventional manner and rinsed in untreated Los Angeles water of the same quality used to prepare the plating solution. The steel panel was then immersed in the above solution and plated in a 267 ml. Hull Cell at 2 amperes for minutes. The deposit was smeared and streaky in the range of l to amps. per square foot and bright above and below this range.

EXAMPLE II EXAMPLE 111 A steel panel was plated in the same manner as set forth in Example I in which the plating solution contained no soluble silicate compound. After cleaning and rinsing in the same quality of water used to form the plating solution, but containing 6 oz./ga1. of sodium metasilicate, the zinc deposit was bright and uniform throughout the range of 1 to amps. per square foot.

EXAMPLE IV A zinc plating solution was prepared using the same water as in Example 1 and containing:

Zinc metal Sodium hydroxide Sodium cyanide A reaction product of diethanol amine, formaldehyde, epichlorohydrin and alpha picoline prepared in accordance with Example 41 of U.S. patent to Kampe 3,655,534

1.2 oz./gal. l0 oz./gal. 1.2 ozjgul.

l5 cc./gal.

A steel panel was cleaned in the conventional manner and rinsed and plated in a 267 ml. Hull Cell using the above solution in the same manner as described in Example I. The zinc deposits showed some streaking and had a smutty appearance in the area of 5 to 10 amps. per square foot.

EXAMPLE V Example IV was repeated with the addition of 0.5 oz./ga1. of sodium orthosilicate to the plating solution which completely eliminated the streaking and smutty appearance of the deposit.

EXAMPLE VI A plating solution was prepared using the same water as in Example 1 containing the following ingredients:

Zinc metal Sodium hydroxide The reaction product of ethylene diamine and epichlorohydrin prepared in accordance with the Example in U.S. patent to Winter 2,791,554 Anisic aldehyde bisulfite 1.0 oz./gal. l0 oz./gal.

7.55 cc./gal. 0.15 g./gal.

0.57 percent by volume of sodium silicate containing about 28.7% SiO was added to the: bath in Example V1. The steel panel was rinsed and plated in the same manner as set forth in Example V1 and the resulting zinc coating showed no sign of smut in the low current density range.

In all of the above examples the zinc was added to the solutions as zinc oxide.

A low cyanide containing alkaline zinc plating solution is herein defined as one which contains less than about 15 grams per liter of free cyanide.

We claim:

1. An alkaline zinc electroplating solution containing water and less than about 15 gms./l of free cyanide, a soluble zinc compound for supplying zinc to be e1ectrolytically deposited on an object, a brightening compound capable of producing a bright or semi-bright zinc deposit but which will produce a dull, smeary, or smutty zinc deposit because of impurities in the water and to which has been added at least about 0.001 oz./- gal. of an alkaline soluble metal silicate to substantially eliminate dull, smeary or smutting deposits.

2. The solution of claim 1 in which the soluble silicate is water glass, sodium disilicate, sodium metasilicate or sodium orthosilicate.

3. In a method for electroplating zinc on an object in an alkaline zinc plating solution in which the plating solution contains water, and less than about gms./l of free cyanide, a soluble zinc compound for supplying zinc to be electrolytically deposited on an object and a brightening agent capable of producing a bright or semi-bright zinc deposit and in which the water contains impurities which will prevent the deposition of a bright or semi-bright zinc deposit on an object, the improvement which comprises adding at least about 0.001 oz./gal. of an alkaline soluble metal silicate to the solution to insure a bright or semi-bright zinc deposit.

4. The process according to claim 3 in which the soluble silicate is water glass, sodium disilicate, sodium metasilicate or sodium orthosilicate.

5. In a method for electroplating zinc on an object in an alkaline zinc plating solution in which the object is cleaned and rinsed just prior to insertion into the electrolytic solution in which the object is to be plated and in which the plating solution contains water and less than about 15 gms./l of free cyanide, a soluble zinc compound for supplying zinc to be electrolytically deposited on an object and a brightening agent capable of producing a bright or semi-bright zinc deposit and in which the rinse water and the water in the electroplating solution contain impurities which will prevent the deposition of a bright or semibright zinc deposit on an object, the improvement which comprises adding an alkaline soluble metal silicate in an amount of at least about 0.001 oz./gal. to the rinse water used to rinse the object just prior to electroplating the object to produce a bright or semi-bright zinc deposit.

i. UNITED S'LA'JfES PATENT OFFICE x g CER'JTIFICA'JTE OF CORRECTION I Pat t" 1: 3 63; l D December 24, 1974 v gy RICHARD B. KESFSLER and FRED I. NOBEL It ,is certified that error appears in the above-identified pat ent and that said Letters Patent are hereby corrected as shown below:

In the Title, "METHODS" should read --METHOD The name of the Assignee is incorrect, "Lea-Ronaf, Inc should read Lea-Ronal, Inc

Column 1, Line 9 "alkaline in" should read ---alkaline zinc- Signed and sealed this 27th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Arresting Officer and Trademarks 

1. AN ALKALINE ZINC ELECTROPLATING SOLUTION CONTANING WATER AND LESS THAN ABOUT 15 GMS./1 OF FREE CYANIDE, A SOLUBLE ZINC COMPOUND FOR SUPPLYING ZINC TO BE ELECTROLYTICALLY DEPOSITED ON AN ABJECT, A BRIGHTENING COMPOUND CAPABLE OF PRODUCING A BRIGHT OR SEMI-BRIGHT ZINC DEPOSIT BUT WHICH WILL PRODUCE A DULL, SMEARY, OR SMUTTY ZINC DEPOSIT BECAUSE OF IMPURITIES IN THE WATER AND TO WHICH HAS BEEN ADDED AT LEAST ABOUT 0.001 OZ./GAL. OF AN ALKALINE SOLUBLE METAL SILICATE TO SUBSTANTIALLY ELIMINATE DULL, SMEARY OR SMUTTING DEPOSITS.
 2. The solution of claim 1 in which the soluble silicate is water glass, sodium disilicate, sodium metasilicate or sodium orthosilicate.
 3. In a method for electroplating zinc on an object in an alkaline zinc plating solution in which the plating solution contains water, and less than about 15 gms./l of free cyanide, a soluble zinc compound for supplying zinc to be electrolytically deposited on an object and a brightening agent capable of producing a bright or semi-bright zinc deposit and in which the water contains impurities which will prevent the deposition of a bright or semi-bright zinc deposit on an object, the improvement which comprises adding at least about 0.001 oz./gal. of an alkaline soluble metal silicate to the solution to insure a bright or semi-bright zinc deposit.
 4. The process according to claim 3 in which the soluble silicate is water glass, sodium disilicate, sodium metasilicate or sodium orthosilicate.
 5. In a method for electroplating zinc on an object in an alkaline zinc plating solution in which the object is cleaned and rinsed just prior to insertion into the electrolytic solution in which the object is to be plated and in which the plating solution contains water and less than about 15 gms./l of free cyanide, a soluble zinc compound for supplying zinc to be electrolytically deposited on an object and a brightening agent capable of producing a bright or semi-bright zinc deposit and in which the rinse water and the water in the electroplating solution contain impurities which will prevent the deposition of a bright or semibright zinc deposit on an object, the improvement which comprises adding an alkaline soluble metal silicate in an amount of at least about 0.001 oz./gal. to the rinse water used to rinse the object just prior to electroplating the object to produce a bright or semi-bright zinc deposit. 